1. Field of the Invention
The invention relates to the fabrication of integrated circuits and to the fabrication of photomasks useful in the manufacture of integrated circuits.
2. Background of the Related Art
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's fabrication plants are routinely producing devices having 0.35 μm and even 0.18 μm feature sizes, and tomorrow's plants soon will be producing devices having even smaller geometries.
The increasing circuit densities have placed additional demands on processes used to fabricate semiconductor devices. For example, as circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to sub-micron dimensions, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Reliable formation of high aspect ratio features is important to the success of sub-micron technology and to the continued effort to increase circuit density and quality of individual substrates and die. High aspect ratio features are conventionally formed by patterning a surface of a substrate to define the dimensions of the features and then etching the substrate to remove the substrate material. Consequently, reliable formation of high aspect ratio features requires a precise patterning and etching of the substrate.
Photolithography is a technique used to form precise patterns on substrates to be etched to form the desired devices or features. Generally, photolithography techniques use light patterns to expose photoresist materials deposited on a substrate surface to develop precise patterns on the substrate surface prior to the etching process. In conventional photolithographic processes, a photoresist is applied on the material to be etched, and the features to be etched in the material, such as contacts, vias, or interconnects, are defined by exposing the photoresist to a pattern of light through a photolithographic photomask which corresponds to the desired configuration of features. A light source emitting ultraviolet (UV) light, for example, may be used to expose the photoresist to chemically alter the composition of the photoresist. The altered or the unaltered photoresist material is then removed by chemical processes to expose the underlying material of the substrate while the retained photoresist material remains as a protective coating. Once the desired photoresist material s removed to form the desired pattern in the photoresist, the exposed underlying material is then etched to form the features in the substrate surface.
Photolithographic photomasks, or reticles, typically include a substrate made of an optically transparent silicon based material, such as quartz (i.e., silicon dioxide, SiO2), having an opaque light-shielding layer of metal, typically chromium, patterned on the surface of the substrate. The metal layer is patterned to form features which define the pattern and correspond to the dimensions of the features to be transferred to the substrate. Generally, conventional photomasks are fabricated by first depositing a thin layer of metal on a substrate comprising an optically transparent silicon based material, such as quartz, and depositing a photoresist layer on the thin metal layer. The photoresist is then patterned using conventional patterning techniques. The metal layer is etched to remove material not protected by the photoresist, thereby exposing the underlying silicon based material.
In order to achieve current circuit densities, alternating phase shift photomasks are being used to increase the precision of the etching pattern formed on the substrate by increasing the resolution of the light passing through the photomask. Alternating phase shift photomasks are fabricated by the same method described above, but with the additional step of etching the exposed silicon based material to form features that refract the light passing therethrough by one half-wavelength. The half-wavelength light has a greater intensity and improved resolution over the unmodified light, thereby allowing the formation of more precise patterns on the underlying substrate. The refraction of light to produce a proportionally shortened wavelength is based on the composition and thickness of the substrate, and the photomask features are etched into the silicon based material to change the thickness of the material the light passes through, and thus change the wavelength of the light. To modify the light to produce the desired wavelength, the etched features formed in the silicon based material of the substrate must be precisely formed in the substrate with a minimal amount of defects in the feature structure.
Current etching processes for silicon based materials, such as those materials used for dielectric layers in semi-conductor manufacturing, have proven unsuitable for etching features in photomasks. For example, the required processing temperatures, or thermal budgets, of materials, such as photoresists, used in photomask fabrication, are lower than the temperatures experienced in conventional dielectric etching processes. If the thermal budget is exceeded during etching of the photomask, the photoresist layer can detrimentally deteriorate, and consequently cause imprecise features to be etched in the underlying silicon based material, resulting in the formation of defective photomasks.
Additionally, current etch chemistries, such as a mixture of CHF3 and oxygen, used to etch silicon based substrates in photomask fabrication have not produced quality photomasks because the chemistry and the processing conditions have not been able to achieve acceptable feature structure. High quality photomasks require features etched in the silicon based material to have straight sidewalls, a flat bottom, and a angle between the sidewalls and the bottom of the feature, which is referred to as a profile angle, between about 85° and about 90°. If the profile angle is formed with unacceptable tolerances, i.e., angles of less than about 85°, the properties of the light passing through the feature may be detrimentally affected, such as having a less than desirable light resolution, and produce less than desired patterning of the underlying substrate.
One difficulty with achieving acceptable feature structure by current etch chemistries and processing conditions occurs when the CHF3 processing gas produces plasma radicals, such as CHF2, which can polymerize and form deposits on the surfaces of the features formed in the silicon based material of the photomask or on the processing chambers surfaces. The polymer deposits may then flake and produce a particle problem in the chamber and in the etched features. Particle deposition in the features can interfere with the etching process and result in imprecisely formed features. Particle deposition in the features after etching can also lead to interference with the light passing therethrough to produce numerous patterning defects in the subsequent photolithograpic processing of substrates.
Polymer deposits may also form on the inner surfaces of the features, and prevent consistent etching of the features, particularly on the bottom and lower sidewalls of high aspect ratio features. In order to etch the silicon based material of the substrate, the etch process first removes any polymer deposits formed thereon prior to etching the underlying silicon based material. The etching interference caused by the deposited polymers, or passivating deposits, can result in features formed with undesirable structures. For example, it has been observed that the current etch chemistries and processing conditions for etching silicon base materials produce features with uneven or tapering sidewalls, feature bottoms which are convex, concave, or have rough surfaces, and profile angles of less than 85°. Such features detrimentally affect the light passing therethrough which can result in imprecise patterning by the photomask.
The polymer deposits may also detrimentally affect the etching rate of the silicon based materials in comparison to the etching rate of the photoresist materials to produce imprecisely formed features. The etching rate, or removal rate, of one material in contrast to another material is often described as the selectivity of the process to the materials. Polymer deposits formed on the surfaces of the silicon material are etched from the surface of the features prior to the underlying silicon, thereby resulting in the silicon material being etched at a slower rate than would normally occur. A lower etching rate of the silicon material having polymer deposits formed thereon in comparison to the photoresist material etch rate corresponds to a lower selectivity. The effect of a lower selectivity is the premature removal of photoresist material which may produce features which are not etched to the desired dimensions, or which may have tapered dimensions or rounded corners at the top and bottom of the feature. The imprecisely formed features can detrimentally affect the resolution and optical performance of light passing therethrough.
Therefore, there remains a need for a photomask etching chemistry and process which limits polymer formation, minimizes defect formation, and forms features with straight sidewalls, flat, even bottoms, and high profile angles. It would also be desirable if the process provided improved etch selectivity.